1. Field of the Invention
The present invention relates generally to a pulse width modulated signal amplifier, and more particularly is directed to improvements in an amplifier of that type which has a stable negative feedback circuit to improve its distortion characteristics.
2. Description of the Prior Art
In the prior art, it is known to use a pulse width modulated amplifier in audio or other amplifiers of the class D type. In such amplifiers, instead of using transistors as variable resistors, transistors are used as switches in the pulse width modulated amplifier. One known method for producing a pulse width modulated signal is to compare a triangular waveform, as a carrier, with the instantaneous level of an input signal as a modulating signal. Another known method does not need an additional carrier to be modulated. For instance, in the latter case, an amplifier with a feedback circuit and higher gain is used to provide the modulating carrier and the input signal to be applied to the amplifier is superimposed on the feedback signal so as to change the ON to OFF ratio of the output signal. Although the last described method for producing a pulse width modulated signal simplifies the construction of the respective amplifier, such method inherently involves rather large distortions. Although it has been suggested to reduce such distortion by providing a feedback that is as large as possible, such large feedback is not an acceptable remedy in that it makes the amplifier unstable by causing oscillation thereof.